Polarized Photoluminescence Emission Research Articles

Chiral Polymer‐Organic Molecule Composite with Circularly Polarized Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence by Bridging Effect of Hydrogen Bond

AbstractCircularly polarized luminescence is essential to chiral and photonic science, but achieving circularly polarized thermally activated delayed fluorescence (CP‐TADF) and circularly polarized room‐temperature phosphorescence (CP‐RTP) simultaneously remains a great challenge. This is because it is difficult to satisfy simultaneously the stable triplet exciton, appropriate energy gap, and the regular chiral environment. Herein, a simple strategy is reported to construct a persistent photoluminescent system, which can achieve TADF and RTP simultaneously by suppressing the non‐radiative transition decay of the triplet exciton through intermolecular hydrogen bonding between acridine flavin (AF) and rigid polymer network. The persistent photoluminescent composite exhibited ultra‐long lifetimes and high quantum yields. Then, a rarely multi‐circularly polarized photoluminescence system containing CP‐TADF and CP‐RTP is constructed by the co‐assembly of cellulose nanocrystals, polyvinyl alcohol, and AF. By utilizing the cholesteric structure, photonic band gap of chiral photonic crystals, and the stabilization mechanism of triplet exciton by intermolecular hydrogen bonding between the light emitter and rigid polymer, chiral photoluminescent films exhibited rare optical properties simultaneously: multi‐circularly polarized photoluminescence emission, high and tunable dissymmetric factor, significant quantum yield, and ultralong lifetimes, which are not reported before and broaden the perspective for multi‐circularly polarized luminescence.

Polarized photoluminescence from Sn, Fe, and unintentionally doped β-Ga2O3

In this work, we demonstrate that β-Ga2O3 shows orientation-dependent polarized photoluminescence (PL) emission and give a comprehensive insight into gallium oxide's PL spectral properties. We characterized the polarization and spectral dependencies of both the incident and emitted light for (−201) unintentionally doped (UID) as well as (−201) and (010) Sn-doped and Fe-doped crystals. We observed for UID and Sn-doped samples that the electron to self-trapped hole and native defect-related emission bands are linearly polarized with polarized emission intensities ordered as E || c (and c*) > E || a (and a*) > E || b. Furthermore, the spectral shape of emission does not change between the UID and Sn-doped samples; instead, the Sn-doping quenches the total PL spectral intensity. For Fe-doped samples, polarized red emission caused by unintentional Cr3+ doping generates emission intensities ordered E || b > E || c (and c*) > E || a (and a*). It is also observed that in some circumstances, for some doped crystals, the PL spectra can show variations not only in intensity but also in spectral shape along different polarization directions. As an example, the PL emission band for emission along c is blueshifted relative to that along a in Sn-doped β-Ga2O3.

Carbon Nanotubes Coupled with Silica Toroid Microcavities as Emitters for Silicon-Integrated Photonics

A light source based on single-walled carbon nanotubes (SWNTs) is one of the promising candidates for a microsized light source on a silicon chip at telecommunication wavelengths in optical communications and optical interconnects. However, SWNT-based light emitters possess the disadvantage of having a very broad emission spectrum. Here, we present an ultranarrow-linewidth photoluminescence (PL) emitter based on a silica toroid resonator, along with SWNTs, on a silicon chip. We simultaneously managed both excitation and emission lights at telecommunication wavelengths on a silicon chip by employing a very simple in-line configuration consisting of a toroid resonator and a tapered fiber for light input and output. Owing to the extremely high Q factor of our silica toroid resonator, we obtained an ultrahigh Q factor (∼2.1 × 104) of C-band PL emission. We also demonstrated strong PL emission under laterally polarized excitation conditions owing to the strong coupling to the toroid resonator, and laterally polarized PL emission can be selectively generated independently of the excitation polarization direction. This SWNT-based PL emitter based on a simple system with a silica toroid resonator can open routes to highly integrated photonics and optoelectronics on silicon-based platforms.

Polarized Electroluminescence Emission in High-Performance Quantum Rod Light-Emitting Diodes via the Langmuir-Blodgett Technique.

Due to their anisotropic structure, quantum rods (QRs) feature unique properties that differ from quantum dots, such as suppression of non-radiative Auger recombination and linearly polarized light emission. Despite many potential advantages, the progress of QR-based light-emitting diodes (QR-LEDs) is left behind due to the difficulty in aligning QRs. In this study, polarized electroluminescence emission is reported in high-performance QR-LEDs by employing the Langmuir-Blodgett (LB) technique. The adoption of the LB technique successfully produces a highly dense and smooth QR film with a high degree of alignment. As a result, the aligned QR films exhibit polarized photoluminescence emission with a degree of linear polarization of 2.1. Advantageous features of the LB technique, such as nondestructiveness, precise thickness control, and the nonnecessity of an additional matrix material, allow to fabricate QR-LEDs with the same procedure as the standard spin coating-based scheme. The device is fabricated via the LB technique, which shows excellent device performance, such as the low turn-on voltage of 1.8V, peak luminance of 56287 cd m-2 , and peak external quantum efficiency (EQE) of 10.33%. Furthermore, these devices clearly exhibit an indication of polarized electroluminescence emission, which opens new opportunities for QRs in display technologies.

Polarimetric microscopy for optical control and high precision measurement of valley polarization.

Two-dimensional transition metal dichalcogenides (2D TMDCs) have two degenerate energy valleys in their Brillouin zone, and these two separate valleys can be used as an information carrier in optoelectronic devices. Circularly polarized optical pumping can selectively populate a single valley, resulting in direct band transitions at the populated valley and associated circularly polarized photoluminescence (PL) emission. However, the birefringence and linear dichroism in optical microscopes can not only distort the circular polarization states of optical pumping but also contaminate experimentally derived information about the polarization states of valley-polarized PL signals. To solve this problem, we suggest a polarimetric microscopy method that manipulates the valley population by optical pumping and measures the polarized PL emission of 2D TMDCs; pumping beam polarization can be precisely controlled by the polarization correction unit, and the original polarization state of the emitted PL signal can be recovered using the Mueller matrix inherent to the optical experimental setup. Using our method, errors when measuring the degree of circular polarization in PL signals can be reduced from 19% to 6%, enabling accurate estimation of exciton lifetimes. Our result offers an accurate and reliable way to process information in valley-based optoelectronic devices.

Field-induced spin splitting and anomalous photoluminescence circular polarization in CH3NH3PbI3 films at high magnetic field

The organic-inorganic hybrid perovskites show excellent optical and electrical properties for photovoltaic and a myriad of other optoelectronics applications. Using high-field magneto-optical measurements up to 17.5 T at cryogenic temperatures, we have studied the spin-dependent optical transitions in the prototype $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, which are manifested in the field-induced circularly polarized photoluminescence emission. The energy splitting between left and right circularly polarized emission bands is measured to be \ensuremath{\sim}1.5 meV at 17.5 T, from which we obtained an exciton effective $g$ factor of \ensuremath{\sim}1.32. Also from the photoluminescence diamagnetic shift we estimate the exciton binding energy to be \ensuremath{\sim}17 meV at low temperature. Surprisingly, the corresponding field-induced circular polarization is ``anomalous'' in that the photoluminescence emission of the higher split energy band is stronger than that of the lower split band. This ``reversed'' intensity ratio originates from the combination of long electron spin relaxation time and hole negative $g$ factor in $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, which are in agreement with a model based on the k\ifmmode\cdot\else\textperiodcentered\fi{}p effective-mass approximation.

Strong Polarized Photoluminescence from Stretched Perovskite‐Nanocrystal‐Embedded Polymer Composite Films

AbstractPolarized light is very necessary to achieve functional optical systems for display, imaging, and information storage. Luminescent materials with polarized emission are of great interest to achieve polarized light. Here, strong polarized photoluminescence from stretched perovskite‐nanocrystal‐embedded polymer composite films is reported by combining an in situ fabrication process with controllable mechanical stretching. The material characterizations show that perovskite quantum dots (QDs) in stretched composite films are oriented aligned into wires along the stretching direction. The optical measurements illustrate that the stretched composite films exhibit not only isotropic absorption but also polarized photoluminescence emission. This feature can be explained with their unique structure of “QD‐aligned wires”. The achieved polarization ratio is consistent with the calculated results by considering the dielectric confinement of optical electric field and exciton–exciton interactions. In addition, the optimized stretched composite films show strong photoluminescence emission with a polarization ratio of up to 0.33 and a quantum yield of 80%. The use of these composite films in liquid crystal display backlights has potential to increase the light transmittance of polarizers from 50% (without considering the optical loss) to 65%, which is of great significance to improve the energy efficiency.

Optical polarization characteristics of c-plane InGaN/GaN asymmetric nanostructures

Highly ordered c-plane InGaN/GaN elliptic nanorod (NR) and nano-grating (NG) arrays were fabricated by our developed soft UV-curing nanoimprint lithography on a wafer. The polarized photoluminescence emission from these elliptic NR and NG arrays has been investigated both theoretically and experimentally. Considerable in-plane optical anisotropy, with a polarization ratio of 15% and 71% and a peak shift of 5.2 meV and 28.1 meV, was discovered from these c-plane InGaN/GaN elliptic NR and NG arrays, respectively. The k·p perturbation theory was adopted to explore this situation, simulating the transitions from conduction subbands to valence subbands and their corresponding optical momentum matrix elements at/around Γ point under the in-plane asymmetric strain. The good agreements of observed and simulated results demonstrate that the in-plane strain asymmetry is the essential cause of the optical polarization in this case, revealing the great potential to utilize strain effect to control the polarization of InGaN/GaN nanostructures.

Analysis of polarized photoluminescence emission of ordered III–V semiconductor quaternary alloys

Ternary and quaternary III–V alloys obtained by metal-organic vapor-phase epitaxy (MOVPE) grow very often with some degree of atomic ordering. Atomic ordering reduces the symmetry of the crystal lattice and thus drastically changes optical properties of the alloy. Moreover, the photoluminescence (PL) emission becomes polarized and its study helps to understand the atomic arrangement within the crystal lattice. In this work we experimentally studied the polarization of the PL emission from different crystallographic planes of several quaternary III–V semiconductor alloys grown on GaAs substrates by MOVPE. We compare the measured PL emission polarization angular patterns with those calculated with a model made for ternary alloys and discuss the limits of application of this model for quaternaries. It is found that the experimentally obtained polarization patterns are consistent with the existence of different ordering crystallographic planes for III- and for V-group atoms.

Impact of the non-planar morphology of pre-patterned substrates on the structural and electronic properties of embedded site-controlled InAs quantum dots

We present an experimental and theoretical analysis of the influence of a surface nanopattern on the properties of embedded InAs/GaAs quantum dots (QD). In particular, we analyze QDs grown on nanoimprint lithography (NIL) patterned grooves and investigate the influence of the non-planar surface morphology on the size, shape, strain distribution, and electronic structure of the embedded QDs. We show that the height reduction of InAs QDs during GaAs capping is significantly less pronounced for the QDs grown on the pattern than for the self-assembled QDs. Furthermore, the pattern has a strong impact on the strain and composition profile within the QD. The experimentally observed strain distribution was successfully reproduced with a three-dimensional model assuming an inverse-cone type composition gradient. Moreover, we show that the specific morphology of the QDs grown in the grooves gives rise to an increase of the vertically polarized photoluminescence emission which was explained by employing 8-band k.p calculations. Our findings emphasize that the surface curvature of the pattern not only determines the nucleation sites of the QDs but also has a strong impact on their morphological properties including shape, size, composition profile, and strain distribution. These properties are strongly cross-correlated and determine the electronic and optical characteristics of the QDs.

Hierarchial Coassembly of a Cyanine Dye in Poly(vinyl alcohol) Fibrous Films by Electrospinning

We report molecular aggregate formation of TTBC (1,1',3,3'-tetraethyl-5,5',6,6'-tetrachlorobenzimidazolocarbocyanine) in submicrometer-sized PVA (poly(vinyl alcohol)) fibers by electrospinning. The formation of the molecular aggregate is examined by solution and instrumental parameters of electrospinning. The precursor solution of PVA/TTBC, in the range of 0.016-0.065 wt % is subjected to electrospinning under an electrical field ranging from 0.95 to 1.81 kV cm(-1). Both randomly deposited and uniaxially aligned fibers are achieved by using two parallel-positioned metal strips as counter electrode. Photoluminescence and polarized Fourier transform infrared spectroscopies are employed to determine spectral properties of the fibers. H-aggregates are formed within the electrospun fibers, regardless of their alignment, and H- and J-type aggregates coexist in the alternative spin-coated and the cast films. A strongly polarized photoluminescence emission is observed in the direction of uniaxially aligned fibers as a result of the orientation of the H-aggregates along the fiber axis. We demonstrate that electrospinning is a process capable of forming and orienting TTBC aggregates during the structural development of the polymer/dye nanofibers. These fibrous films may potentially find applications in optics and electronics.

Highly Oriented Nanofibrils of Regioregular Poly(3‐hexylthiophene) Formed via Block Copolymer Self‐Assembly in Liquid Crystals

AbstractA novel method making use of block copolymer self‐assembly in nematic liquid crystals (LCs) is described for preparing macroscopically oriented nanofibrils of π‐conjugated semiconducting polymers. Upon cooling, a diblock copolymer composed of regioregular poly(3‐hexylthiophene) (P3HT) and a liquid crystalline polymer (LCP) in a block‐selective LC solvent can self‐assemble into oriented nanofibrils exhibiting highly anisotropic absorption and polarized photoluminescence emission. An unusual feature of the nanofibrils is that P3HT chains are oriented along the fibrils' long axis. This general method makes it possible to use LCs as an anisotropic medium to grow oriented nanofibrils of many semiconducting polymers insoluble in LCs.

Optical method of estimation of degree of atomic ordering within quaternary semiconductor alloys

It is well known that within metal-organic vapor-phase epitaxy grown semiconductor ternary alloys atomically ordered regions are spontaneously formed during the epitaxial growth. This ordering leads to bandgap reduction and to valence bands splitting, and therefore to anisotropy of the photoluminescence emission polarization. The same phenomenon occurs within quaternary semiconductor alloys. While the ordering in ternary alloys is widely studied, for quaternaries there have been only a few detailed experimental studies of it, probably because of the absence of appropriate methods of its detection. Here, we propose an optical method to reveal atomic ordering within quaternary alloys by measuring the photoluminescence (PL) emission polarization. The measured and calculated angular dependencies of the polarized PL emission intensity from (100) surface for two semiconductor alloys: Ga0.51In0.49P and (Al0.3Ga0.7)0.5In0.5P are compared in order to estimate the degree of atomic ordering within these alloys. The method shows that the quaternary (Al0.3Ga0.7)0.5In0.5P alloy is a highly ordered structure with the value of the atomic ordering degree close to 0.5.

Color Tuning of Nanofibers by Periodic Organic–Organic Hetero-Epitaxy

We report on the epitaxial growth of periodic para-hexaphenyl (p-6P)/α-sexi-thiophene (6T) multilayer heterostructures on top of p-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the p-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and p-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of p-6P spacer thickness (Δdp–6P) we have determined a critical value of Δdp–6P ≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and p-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from p-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/p-6P multilayer structures depends on the p-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue via white to the yellow-green spectral range.

Spectroscopic anisotropy in laterally self-aligned quantum dots

The spectroscopic features observed in laterally self-aligned InGaAs/GaAs quantum dots (QDs) are compared with those in isolated QDs. Though each aligned QD keeps an individual appearance analogous to that of an isolated QD, the aligned QD shows a unique spectroscopic anisotropy attributed to coupling. The polarized photoluminescence (PL) intensity parallel to the alignment of the QDs is distinctively enhanced, which supports the anisotropic arrangement in a preferential [110] direction as shown by the atomic force microscopic image. The temperature dependence of the PL emission energy is exceptionally redshifted, in contrast with the excitation-power dependence that shows a blueshift behavior, which may possibly be explained by the carrier redistribution in anisotropically coupled QDs.

Supramolecular structure of columnar liquid crystalline π-conjugated oligothiophenes with highly polarized photoluminescence properties

It is possible to control the alignment of columnar liquid crystalline π-conjugated systems by controlling photoluminescence (PL) properties to yield a polarized PL effect by shear stress. A polarization ratio of 11.2 at the fluorescence peak was obtained using columnar liquid crystalline π-conjugated oligothiophenes [T. Yasuda et al., Adv. Funct. Mater. 19, 411 (2009)]. We have therefore revealed the directional distribution of oligothiophene molecules inside the columns, which is the origin of a highly polarized PL emission, through detailed analysis of the dependence of PL polarization ratio on the detection polar angle, assuming that such PL emission is due to the summation of all dipole radiations from oligothiophene molecules in the measured area of the sample.

Optical Properties of Single CdS Nanosheets

We study the optical properties of single CdS nanosheets using micro-photoluminescence and time-resolved photoluminescence (PL) techniques. At low temperature, we observe high quantum efficiency and strongly polarized PL emission, indicating high-quality single-crystal nanosheet growth. Photoluminescence from A and B exciton states is observed with different spatial distributions, suggesting the existence of a variation in strain in the nanosheets. Time-resolved photoluminescence measurements show the exciton lifetime to be ∼230 ps, longer than that of the near-band-edge emission, but shorter than the lifetime of defect-related emission in single CdS nanowires.

Ultrafast exciton dynamics in isolated single-walled nanotubes

Ultrafast relaxation dynamics of photoexcitations in semiconducting single-walled carbon nanotubes (S-NT) isolated in D 2O solution have been investigated using polarized pump–probe photomodulation (with 150 fs time resolution), and cw polarized photoluminescence (PL). Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands were observed, which also showed photoinduced dichroism. After taking into account the PB spectral shift, then the PA and PB bands decay together in time following a power law of the form ( t) − a with a in the range of 0.7–1. The PL emission shows polarization degree that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in S-NT are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime in isolated NTs in solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation.

Exciton dynamics in single-walled nanotubes: Transient photoinduced dichroism and polarized emission

Ultrafast relaxation of photoexcitations in semiconducting single-walled carbon nanotubes ($S$-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed thin NT films and ${\mathrm{D}}_{2}\mathrm{O}$ solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. The PA and PB decay dynamics as a function of time, $t$, follow a power law, ${(t)}^{\ensuremath{-}\ensuremath{\alpha}}$ with $\ensuremath{\alpha}$ in the range of 0.7 to 1. Whereas the PA bands in $S$-NTs in solution uniformly decay, the PB bands, in contrast, have different decay dynamics across the spectrum, which originates from an ultrafast spectral shift. Nevertheless the dynamics of the PA and PB bands for NTs in solution are the same when the spectral shift is accounted for, indicating a common origin. In addition $S$-NTs in ${\mathrm{D}}_{2}\mathrm{O}$ solution show polarized PL emission bands in the mid infrared spectral range that follow almost exactly the infrared absorption peaks of the isolated NTs, as well as their transient PB spectrum. The PL emission shows a degree of polarization that agrees with that of the transient photoinduced dichroism. We therefore conclude that the primary photoexcitations in $S$-NTs are not free carriers, rather they are excitons that are confined along the nanotubes. We found that the transient relaxation kinetics of the excitons depend on the NT form. The fastest exciton dynamics (with sub-picosecond lifetime) characterizes the annealed film, whereas the slowest dynamics (with lifetime of tens of ps) characterizes the isolated NTs in ${\mathrm{D}}_{2}\mathrm{O}$ solution. From the polarization memory decay we could estimate the diffusion constant, $D$, and the diffusion length, ${L}_{D}$, of the excitons along the nanotube. For the annealed films at room temperature we found $D\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and ${L}_{D}\ensuremath{\approx}100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. From the average PL polarization degree, which remains constant across the PL spectrum, and the transient polarization memory decay, we estimate the PL lifetime in NT solution to be of the order of 500 ps. This relatively long PL lifetime is dominated by nonradiative decay processes, which when coupled with the minute PL emission quantum efficiency indicates a very small radiative recombination rate. The weak radiative transition strength is consistent with recent excited state calculations that include electron-hole interaction, which predict that excitons in NTs are basically dark.

Completely polarized photoluminescence emission from a microcavity containing an aligned conjugated polymer

We present a study of 1-D optical microcavities based on a dielectric mirror, a layer of an aligned fluorescent conjugated polymer (poly(9,9-dioctylfluorene) (PFO)) and a metallic mirror. The aligned polymer film is birefringent, with a refractive index difference calculated to be 0.36 between directions parallel and perpendicular to the alignment direction. Because of the large birefringence, two cavity modes are observed within the microcavity stop band. They are split by 32 nm and are polarized parallel and perpendicular to the alignment direction. Photoluminescence from the PFO film correspondingly shows two emission bands with mutually perpendicular polarizations. The polarization ratio of the most intense mode exceeds 300:1. We discuss how this structure might be used to construct highly polarized light-sources.